This section provides background information related to the present disclosure which is not necessarily prior art.
Traditionally, a photovoltaic inverter is operated at the Maximum Power Point (MPP) in order to maximise the revenue from the solar panel plant. For example, in US 2013/0155735, there is shown in FIG. 2B a control system which controls the inverter so that the photovoltaic array voltage is held at the optimum level as required by the MPP. The integral controller (PI control) and limiter are part of the internal control arrangement of the inverter which allows it to follow the required MPP voltage.
Recently, however, there have been cases in which inverter output power needs to be restricted to below the MPP. For example, this may be because the power company has issued a command to restrict the output power of the plant, as the power system may not accept the whole power output of the plant, or for other operational reasons. In other cases, when the sun is strong, the array power output may exceed the agreed maximum export rating of the plant—this may occur if the array is deliberately over-sized relative to the inverter, and by limiting the power, such that the power varies less throughout the day. This requirement is becoming more common with the reduced capital cost of photovoltaic panels, and in parts of the world where real estate allows for the array to be over-sized relative to the rated exported power.
However, when one tries to restrict the power output of the inverter to the grid, the voltage of the array rises. Depending on the system design and the array temperature, the voltage might become too high for the inverter to run, and may rise so high that the inverter cannot operate. Conventional techniques used to address this issue include:
(a) allowing the voltage to rise and accepting the cost of a higher voltage rated inverter, or otherwise re-scaling the system to reduce the voltage, both of which result in higher cost;
(b) using an additional resistor to dissipate the unwanted power with a controller such as a “chopper” (see for example US 2010/0275966)—which is expensive and requires a facility to dissipate the heat energy generated in the resistor, and is therefore not practical in a large system.
There is therefore a need in the art for a more cost-effective and efficient means of allowing restriction or limitation of the output power of an inverter.